Add the Workhorse pages to left nav

• Link to the merge request of the new page: gitlab!84115 (merged)
  • https://docs.gitlab.com/ee/development/workhorse/index.html
  • https://docs.gitlab.com/ee/development/workhorse/channel.html
  • https://docs.gitlab.com/ee/development/workhorse/configuration.html
  • https://docs.gitlab.com/ee/development/workhorse/gitlab_features.html
  • https://docs.gitlab.com/ee/development/workhorse/new_features.html
• Give the MR a descriptive title.
• Link to the merge request that introduced the new page.

Related to gitlab#354845 (closed) where the overarching work is being tracked.